Quarkonium Suppression from SPS to RHIC (and from p+A to A+A)

Heavy quarkonium production is expected to be sensitive to the formation of a quark gluon plasma (QGP). It was (and still is with ongoing data analyses) extensively studied at the CERN SPS, at collision energy of the order of 20 GeV. An anomalous suppression was clearly observed. The PHENIX experiment at RHIC has presented preliminary results that exhibit a similar amount of J/ψ suppression, at ten times higher collision energy. I review the results obtained at both facilities. While interpreting and comparing them, the importance of understanding normal nuclear effects is emphasized. A new method to derive a reference for Au+Au collisions from the centrality dependence of d+Au measurements at RHIC is presented.